I assume the original debt was the subject of a County Court Judgement which then resulted in the charging order. As the amount was less than £5000 I think (but am not certain) that they cannot add statutory interest (which is interest set at the Court rate - currently 8% pa compound).
If I am right - & you should be able to confirm this by talking to the Court office where the judgement was obtained - then I suspect they can only add interest if the original contract provided for them to be able to do so. And the rate of interest should have been set out in the contract, or at least the basis on which it would be calculated (eg it might have said interest would be x% above bank base rate).
You need to check this & do some calculations to see whether the amount of interest is right (assuming they are correct to be adding any at all).
You could try to avoid doing this yourself by asking the agents to provide you with a detailed breakdown of how the current amount is made up - including the interest rates used; & also asking them for proof (a copy of the original contract if applicable) that they can charge interest. However, I don't think they are under any obligation to give you all this information so they may not co-operate.
You should also check the charging order document - that may have something about interest in it.
If, at the end of all this, you are liable for more than the £1500 then you could try negotiating with them but they may well refuse any offer because I presume they know they will get the full amount eventually when your house is sold.